In the manufacture of cushions and other similar articles, such as cushions for vehicle seats, chairs and the like, many widely varying methods have been developed for making such articles. Two commonly known methods of making seat cushions are referred to in the industry as cut-and-sew, where a fabric seat cover is fitted onto a preformed foam core, and "foam-in-place," where the seat cover fabric is drawn by a vacuum into a mold three dimensionally forming the fabric and foam is poured into the mold over the formed fabric.
In cut-and-sew seat cushion manufacturing, the seat cover material is first cut to a certain size and shape to enable it, after being sewn, to either (1) fit over a preformed foam core or (2) be stuffed with foam or another filler material. However, cutting the seat cover material to size and sewing it so it will snugly fit over a preformed foam core requires a great many manufacturing operations to be performed which can result in higher than desired manufacturing costs and a lower than desired rate of seat cushion production. Moreover, in most instances, much of the sewing that is performed in constructing these seat covers cannot be done with the fabric flattened, adding to the difficulty of making cut-and-sew seat covers. Finally, many of these operations must be done manually further increasing production costs.
In many instances, an adhesive is applied over the preformed foam core before the seat cover is fitted onto the blank in an attempt to prevent the cut-and-sew cover from undesirably sliding on the foam core. Unfortunately, unless the application of adhesive is relatively uniform and the bond between the fabric and foam strong and long lasting, there can still be some slippage between the cover and foam causing the foam blank and cover to undesirably wear.
To further minimize slippage and improve the durability of the seat cushion, low stretch, woven fabrics have been used to construct cut-and-sew seat covers. Although, these types of fabrics provide superior durability over other types of seat cover materials, such as higher stretch, knit fabrics, they can be difficult to cut and sew to obtain the appropriate shape to consistently and uniformly fit over preformed foam cushion cores in a high volume, assembly line setting. If fit is poor, wrinkles can form on the cover resulting in a poorly looking seat cushion. Also, if even small wrinkles form, there can be slippage between the cover and foam causing undesirable wear. Although applying an adhesive onto the foam can help better anchor the cover to the foam while preventing wrinkles, it is a costly additional manufacturing step that can also undesirably slow seat cushion production.
As a lower cost method of making seat cushions, foam-in-place seat cushion manufacturing was initially used with vinyl as the outer seat cover material. In this seat cushion manufacturing process, a generally flat piece of vinyl seat covering material is placed in a frame. The frame is placed in an oven where it is heated to soften it sufficiently for being shaped into a cushion cover. To form the vinyl covering, the heated vinyl is placed in a vacuum mold where it is drawn against the inner surface of the mold, conforming the vinyl to its contour. Upon forming, a foam is poured into the mold over the vinyl. When sufficiently cooled and after the foam cures and hardens, a fully formed seat cushion is removed from the mold and vinyl along the bottom edge of the cushion is trimmed.
Unfortunately, vinyl is not very breathable causing perspiration on a person sitting on a vinyl cushion to become trapped between the vinyl and the body of that person. As a result, sitting on seats that have vinyl covers for long periods of time can be uncomfortable. Moreover, vinyl is less durable as it can become brittle and crack over time as plasticizers in the vinyl evaporate or migrate from the vinyl. Therefore, as a result of at least these characteristics, vinyl has been, for quite some time, a less than an ideal choice for seat manufacturers.
In an effort to improve upon at least some of the drawbacks of vinyl, a method of foam-in-place seat cushion manufacturing has been developed that utilizes a fabric seat cover that is highly stretchable. To construct a seat cushion using high stretch cloth fabric as a seat covering material, a laminate of fabric, foam and film is secured in a frame. After preheating, the frame is placed over a vacuum mold where the fabric and film are drawn against the mold by suction with the film acting as a seal for preventing leakage during molding. Thereafter, a foam is poured into the mold over the film and fabric to form the cushion. When the foam has solidified, the cushion is removed from the mold and the fabric and film are trimmed from around the bottom of the cushion.
During vacuum molding, since the fabric is fixtured to the frame, as it is drawn into the cavity of the mold it stretches a great deal. Since the fabric, foam and film are already bonded to form a laminate before forming occurs, there can be only very limited relative movement between the fabric and film during forming to facilitate forming. Should excessive relative movement occur during forming, the foam layer could undesirably tear or rupture also producing wrinkles or another unwanted defect.
During forming, the high stretch fabric is typically never stretched so far as to cause compression of any portion of the fabric, thereby preventing wrinkling. Therefore, this seat cushion manufacturing process requires a seat covering material that can stretch a considerable amount without rupturing, such as would occur if more durable, low stretch fabrics typically used for cut-and-sew seat covers were utilized.
High stretch fabrics suitable for this vehicle seat cushion manufacturing process include knit fabrics, such as circular knit fabrics, and other fabrics possessing high stretch content. Unfortunately, while these high stretch fabrics are used because they are easy to successfully form, they possess relatively poor durability as compared to the relatively low stretch fabrics used in cut-and-sew seat cushions. For example, some of these high stretch fabrics are known to fail during scrub testing after as few as 20,000 scrub cycles whereas lower stretch fabrics are known to be more durable. As a result, seat cushions constructed using high stretch fabrics tend to be less favored by purchasers of seat cushions, such as seating and vehicle manufacturers, as well as owners of products having such seat cushions.
Additionally, even high stretch seat cover fabrics can wrinkle during vacuum molding because they are clamped about their periphery to the frame causing some parts of the clamped fabric to be pulled more than other parts when the vacuum draws the fabric against the mold. This is at least in part because of the heretofore unrecognized effects of compression on a fabric seat cushion blank. These wrinkles can adversely affect the seal between the film and mold which can lessen the vacuum in the mold cavity causing the fabric to be less tightly drawn against the inner contoured surface of the mold cavity than desired. These wrinkles can adversely affect vacuum integrity by functioning as air passages allowing outside air to enter the mold, which even further reduces the integrity of the vacuum also causing the seat cover fabric to be drawn less tightly against the contoured inner mold cavity surface than desired. As a result, after the foam is poured and solidified, these wrinkles and indentions in the fabric where the seat cover was not firmly against the mold can be permanent resulting in a seat cushion of poor appearance and which may not pass quality control. Of course, if a seat cushion having these defects fails quality control inspection, it may have to be repaired at considerable additional expense or, even worse, scrapped.
Another disadvantage of this seat cushion manufacturing process is that a piece of fabric much larger than the seat cushion is required so that it can be clamped in a frame that is also larger than the cushion that is being constructed. This extra fabric, that is the fabric in excess of what is necessary to cover the seat cushion, must be trimmed after the poured foam has solidified resulting in a costly extra manufacturing step, as well as wasting seat cover fabric material.
A somewhat similar foam-in-place seat cushion manufacturing process is disclosed in Sanson, U.S. Pat. No. 4,046,611. In Sanson, a cloth seat covering fabric coated with an elastic plastic coating is fixtured in a frame so that a die can press against the cloth to prestretch before it is vacuum molded. To further form the cloth, the cloth is removed from the frame and placed over a vacuum mold where the cloth is tightly clamped between a gasket and an outer surface of the mold before vacuum is applied to draw the prestretched seat cover cloth against the mold. After forming is complete, foam is poured into the mold over the seat cover cloth and a lid is placed over the gasket and mold to assist in curing and solidifying the foam.
In addition to the aforementioned foam-in-place seat cushion making drawbacks, this process requires more manufacturing steps making it more costly. Moreover, this method is also limited to fabrics that can be greatly stretched because the seat cover fabric is clamped to the mold during the steps of prestretching and vacuum forming requiring it to stretch a great deal before it contacts the mold. Additionally, this method is also limited to high stretch fabrics because of the elastic plastic coating applied to the fabric.
Another somewhat less similar foam-in-place seat cushion manufacturing method is disclosed in Lischer, U.S. Pat. No. 4,247,348. The method disclosed in Lischer requires a rather complex and more expensive multi-layer seat cover cloth of special construction having its outer layer constructed of a fabric that cannot have been heat set before vacuum forming. The use of fabric that has not been heat set is required by this method to maximize the elasticity of the fabric outer layer during forming so that it will be able to stretch sufficiently to conform to intricate mold patterns.
In Lischer, the preferred seat cover cloth blank is a laminate that has an outer layer of fabric sandwiching a layer of foam between the fabric and a film layer. The foam layer is provided to permit some minute or very limited movement of the fabric relative to the film for preventing rupture of the film during forming. To construct the cloth, a rather complicated and relatively costly process is required to bond the layers together.
In the molding process disclosed in Lischer, the cloth is first cut to size and then heated to a temperature greater than the temperature at which the cloth will heat set and the film loses its shape memory. After preheating, the cloth is placed in a heated female vacuum mold where a vacuum is applied to draw the cloth against the inner surface of the mold. To set the cloth, the mold is also maintained for a period of time at a sufficiently high temperature to enable the cloth to become heat set after it is sufficiently cooled. Thereafter, the mold and cloth are allowed to cool approximately 30.degree. to 50.degree. Fahrenheit so that the fabric does become set. After the mold has cooled and the cloth become heat set foam, in its liquid state, is poured into the female mold over the cloth. Unless the mold is cooled or the cloth transferred to a cooler mold, the cooling of the cloth and mold can be time consuming reducing production volume.
To cure the foam, the temperature of the mold must be lowered to approximately 100.degree. Fahrenheit or else the foam and cloth must be thereafter disadvantageously transferred to a cooler female mold. During expansion of the foam, a male mold is placed over the top of the foam and female mold. Unfortunately, this method requires special cloth of more complex laminated manufacture which can increase the cost of making seat cushions using this method. Additionally, this method of manufacture is also more complicated and significantly more time consuming making it more costly and less desirable to practice in a high volume, manufacturing setting. Moreover, if too great of relative movement occurs between the film and fabric during forming, the foam can rupture producing a finished product that is not firmly adhered to its base. If not fly adhered to its base, wear by rubbing can occur reducing the durability of the fabric while also possibly adversely affecting its appearance.
Finally, a heretofore unrecognized problem in forming fabrics, and particularly low stretch fabrics, is the role that stress plays during forming. A recognized difficulty in forming low stretch fabrics is that they previously could not be deeply drawn into molds because they could not stretch a great deal during forming or else they would wrinkle. Previously believed to be unrecognized is the role that tensile and compressive stresses played in the formation of these wrinkles in low stretch fabrics. It was previously not believed to be recognized that certain areas of a fabric blank are subjected to compressive stresses during forming which leads to buckling of the fabric in those high compressive stress regions which, in turn, causes wrinkling of the fabric, all of which is highly undesirable. As previously discussed, wrinkling is undesirable because it results in a poorly looking cushion and one which can have poor wear characteristics.